He was made a Cardinal at the age of 23 and assisted the Pope in administering the affairs of the Holy See and in governing the Church. Soon thereafter he was made Archbishop of Milan.
His endeavors on behalf of the 19th Ecumenical Council of Trent (1545-1564) were especially meritorious and fruitful.
He helped to direct and guide it and bring it to a successful conclusion.
He then proceeded to enforce its decrees in the Archdiocese of Milan and thoroughly reformed Catholic life in his See.
During a plague he walked barefooted in the public streets, carrying a cross, with a rope around his neck, offering himself as a victim to God for the transgressions of his people.